Glazing for Daylight
Low-emissivity (low-e) coatings were developed in the early 1970s and introduced into the market around 1980. Emissivity is the ability of a surface to emit radiant energy; part of the energy absorbed by the glass is radiated away from the glass surface.
Most low-e coatings have high VT and reflect 40 to 70 percent of infrared radiation. Low-e coatings offer a reduction of 5 to 37 percent in ultraviolet radiation. Low-e coatings on glass or plastic films can be applied in several ways and are categorized into two major classes, depending on their application during manufacturing and their assembly within a window system.
Protective glazing is a form of coated glazing and is often used in warm climates. The surfacing, made up of metallic particles, reflects primarily visible radiation and darkens the building interior. The designer should look carefully at the SHGC of this type of glazing before specifying. Sometimes so much solar radiation is reflected from buildings using mirrored glass that the heat load on adjacent buildings is increased.
The installed performance of a low-e coating varies in relation to the climate and placement of the coating within the glazing unit. Low-e coatings are applied to one glazing surface facing the air gap of an insulating gas (IG) unit. The location of this surface does not affect the thermal conductivity, but does affect the solar heat gain properties.
It is important to note that these coatings are available in a wide assortment of ratings. The designer should review performance values to determine which are the most appropriate for each particular project. Ratings provided by the National Fenestration Rating Council (NFRC) may be useful in comparing overall performance values.
Spectrally selective low-e coatings are made to be selective to specific wavelengths of light. Advanced forms of low-e coatings are being developed that further select the transmission of visible light and reflection of solar infrared and can block up to 99 percent of the ultraviolet radiation. Many are currently available and are used in assemblies called "superwindows." Use of these technologies will provide greater freedom in fenestration design and glazing area and reduce the need for perimeter heating equipment.
Reflective materials can be deposited on glass in many ways and are used on clear, tinted, or otherwise treated surfaces. Apart from some higher performance versions, most reflective coatings do not require excessive protection from damage.
Reflective coatings reflect visible light and infrared radiation. They generally reduce visible-light transmission more than infrared energy, which gives them a poor daylight Ke (about 0.5). Like mirrored glazing, they can cause secondhand heat and glare and raise the cooling costs of adjacent buildings. Just as these coatings reflect light to the exterior during the day, they reflect interior light at night due to their low VT and high reflectance.
Directionally Selective Materials
Directionally selective materials reject or redirect incident solar radiation based on a geometric relationship between radiation and the material. These glazings can redirect light to a predetermined location. They include glass blocks, silk-screened glazings, prismatic devices, enclosed louvers, holographic films, and embedded structures.
Frit is the most common angle-selective coating. It consists of a ceramic coating, either translucent or opaque, which is screen printed in small patterns on a glass surface. The pattern used on the glass controls the light based on its angle of incidence. The color of frit controls the reflection or absorption and the control of view or visual privacy.
Visual transparency can also be controlled by applying frit to both sides of the glass so that at some angles it appears transparent, and at other angles it appears opaque. Angle-selective materials can be thought of as a series of fins or overhangs within a piece of glass that filter or block light.
Prismatic systems redirect light by refraction but use dielectric (nonconducting) materials. Refraction is the passing or reflection of light, which produces parallel bands of light. Fresnel lenses, made of microscopic prismatic materials embedded within the glass, are a common type of prismatic device.
Depending on the application, Fresnel lenses can focus light inward or outward and can work the way water appears to bend a partially immersed pencil. Prismatic systems other than Fresnel lenses have limited commercial use.
Switchable optical windows, or smart windows, can change their physical properties based on predetermined conditions. These chromogenic glazings can be altered either passively or actively. Where a change is desired, switchable materials can provide glare reduction, privacy, daylight and solar control, and reduction of ultraviolet transmission. Most chromogenic glazings are still in the development stages and not yet available for large-scale commercial projects.
The ideal material in a hot climate would, on command, transmit only visible light, modulate the intensity of the light, and then distribute it evenly within the space. When combined with continuously dimming controls, switchable materials can provide these significant benefits and save energy in commercial buildings. Energy simulations of office buildings indicate that smart windows with lighting controls in arid climates can provide 30 to 40 percent energy savings over conventional windows.
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Gregg D. Ander, FAIA is chief architect of Southern California Edison. He has authored more than 70 technical and design-related articles and received numerous awards. He also teaches advanced environmental controls at the Department of Architecture at California Polytechnic University at Pomona.
This article is excerpted from Daylighting Performance and Design, copyright © 2003, available from John Wiley & Sons and at Amazon.com.